Phase detection circuits have been used to detect the relative phase of two input signals. Some of these circuits have converted the analog input signals into digital signals through threshold triggering at some preselected signal amplitude value. Further processing and filtering of the resulting signals was used to provide an output signal which was indicative of the relative phase of the two input signals. These prior techniques were limited to their usefulness because of their poor linearity particularly in the dead band region, i.e., that region where the two signals were very close in phase.
In accordance with the preferred embodiments of the present invention, two input signals having an unknown phase relationship are input to threshold trigger circuits which provide two digital signals in response thereto. These digital signals are input to an interactive flip-flop register. This interactive register provides an output signal whose duty cycle corresponds to the phase difference of the input signals. This output signal is then filtered to provide a DC output signal which varies between two predetermined voltages in correspondence with the relative phase difference. The preferred embodiment thus provides improved linearity with a very narrow dead band. Contributing to this improvement is the fact that this invention allows the logic circuits to operate at one-half the input frequency.